1. Field of the Invention
The invention relates to dimming control methods and lighting systems incorporating dimming control; and more particularly to a dimming control method combining a continuous dimming control technique and a burst dimming control technique, and a lighting system employing this kind of dimming control method.
2. Background of the Invention
The improved quality and reduced costs of modern Liquid Crystal Displays (LCDs) are making the LCD an increasingly popular choice in the field of display devices. LCDs are traditionally employed in notebook computers and other portable computer systems. LCD devices have made progress in conjunction with the evolution of computer displays from the conventional Video Graphics Array (VGA) standard to the newer Extended Graphics Array (XGA) standard. Nowadays, LCD devices have a superior display quality to that of Cathode Ray Tubes (CRTs), and are poised to replace conventional CRT devices.
The LCD cannot achieve light-emission independently—it has to rely on a backlight source. The backlight source and relevant elements are indispensable in the direct-viewing type of LCD device. The performance of the backlight source significantly influences the display quality of the LCD device. Moreover, the backlight source is a large contributor to the cost and power consumption of the LCD device.
Dimming control of the backlight source is nowadays performed by way of either of two techniques; namely, a continuous dimming control technique or a burst dimming control technique. In the continuous dimming control technique, an amplitude of a current output to a light-emission unit is adjustable according to a change of brightness of the light-emission unit, and the output current is a continuous wave.
FIG. 4 is a graph showing a relation between the amplitude of the current output to the light-emission unit and the brightness of the light-emission unit in the continuous dimming control technique according to a typical prior art dimming control method. The horizontal abscissa represents the brightness of the light-emission unit, and the vertical ordinate represents the amplitude of the current output to the light-emission unit. Amax is the amplitude of a maximum current output to the light-emission unit, and Amax corresponds to a maximum brightness Bmax of the light-emission unit. A0 is a leakage current, and may occur when the current is input to the light-emission unit. When the amplitude of the current output to the light-emission unit is diminished to the value of A0, the light-emission unit has no current flowing therethrough if the leakage current A0 is fully taken into account. Thus, one end of the light-emission unit may emit light while the other end may not emit light. In order to maintain the lighting quality of the light-emission unit, generally the backlight source may be set with a lower-limit amplitude of the current Amin which is higher than the leakage current A0, while the brightness of the light-emission unit is set as a predetermined minimum brightness Bmin of the light-emission unit. Thus the leakage current A0 is a main drawback of the continuous dimming control technique, because it can result in the current output to the light-emission unit being insufficient. In addition, the brightness of the light-emission unit may also be insufficient.
In the burst dimming control technique, the brightness of the light-emission unit is adjusted by discontinuously activating the light-emission unit. That is, a current of fixed amplitude can be output to the light-emission unit, but a duty cycle of the current output to the light-emission unit is adjusted according to variations in the brightness of the light-emission unit. When the brightness of the light-emission unit is high, the duty cycle of the current output to the light-emission unit is adjusted to be high (the highest value is 1). On the other hand, when the brightness of the light-emission unit is low, the duty cycle of the current output to the light-emission unit is adjusted to be low.
In contrast to the continuous dimming control technique, the burst dimming control technique is better insofar as the leakage current can be eliminated by fixing the amplitude of the current output to the light-emission unit. Even if the duty cycle of the current output to the light-emission unit is adjusted to an extremely small value, that is the average current is very low, the light-emission unit can still emit light uniformly. In the burst dimming control technique, because an average current ratio output to the light-emission unit is higher than that in the continuous dimming control technique, the brightness of the light-emission unit is sufficient. However, in the burst dimming control technique, the light-emission unit is discontinuously activated so that the backlight source has a higher noise than the continuous dimming control technique. If a relatively low quality power source is employed in the LCD device in order to reduce costs, this itself may lead to higher noise of the backlight source. In such case, the relatively high noise inherent in the burst dimming control technique may render this technique unsatisfactory.